UCC28750: Design Clarifications

Hi Punith,

Thank you for reaching out.

For UCC28750 we only have SIMPLIS model. No TIN/ SPICE model is available.

This IC will require an optocoupler for feedback. This does not support feedback via aux winding. We have other controllers like UCC28700/10/30 etc. which support feedback with aux winding.

For transformer design, the excel tool only can estimate the required inductance and turns ratio. If using multiple winding, I would advise to calculate considering the total system power. (i.e include the effect of all three windings) to get the equivalent current referred to one winding. In that way, it would be possible to estimate the Ipeak which will be required for transformer design. 

Please let us now if you have further questions.

Regards,

Harish

Hi Harish,

Thanks for your response.  We will look into your inputs and revert.

Meanwhile, we had another query as below,

How is UCC28750 compensating for the large variation in the optocoupler Current Transfer Ratio?

Thanks,

Punith

Hi Punith,

Thank you for reaching out.. Ideally if the optocoupler is designed considering the minimum value it should ensure stability. The following app note should how this is done.

https://www.we-online.com/components/media/o760916v410%20ANP113a%20EN.pdf

https://www.ti.com/lit/an/sluaa66/sluaa66.pdf?ts=1725382391851&ref_url=https%253A%252F%252Fwww.google.com%252F

Ideally the loop needs to stable for the variations in CTR.

Regards,

Harish

Hi Harish,

Thanks for your reply.

We have further evaluated UCC28750 and trying to understand the workings of this IC through the eval board UCC28750EVM-071

According to our understanding the feedback by optocoupler is taken by generating a reference voltage on the cathode (pin 2) of the optocouple and the secondary output at the anode (pin 1) . There will be a change in the drop across the optocopler led which will modulate the intensity / current across the optocoupler . which inturn is controling the current draw and volage on the feeedback pin .
when we were taking hardware waveforms of the optocoupler We observed the following.
1) Vref pin of TLV431 = 2.48V
2) Optocoupler anode (pin1) = 13.2V
3)Optocoupler cathode (pin2) = 12.9V (Reference voltage)

According to the TLV431 datasheet we were expecting around 24V reference voltage but are observing 13.2V

Can you let us know more about this observation?

This was the other observation as below,

With 25W Load,when taking waveforms at the Opto coupler input and output, we saw two frequencies, one is 37KHz which is the switching frequency at that load. A second frequency of 250Hz which is a ramping up and down was also present.

We are unable to relate to the source of the 250Hz signal. Can you let us know, if similar outputs are observed and the reason behind it?

Thanks & Regards,

Punith

Hi Punith,

1. The voltage at the cathode of TL431 should be Vout - Iled x Rled - Vf(opto). Please refer compensation with TL431 which provides some insight into this.

http://powersimtof.com/Downloads/PPTs/Chris%20Basso%20APEC%20seminar%202011.pdf

2. The 250hz might be due to loop compensation issues - could be due to choice of Rled resistor wrt Rpull and compensation netweork around TL431.

Regards,

Harish

Hi Harish,

Thanks for your response.

We have further queries as below,

The Design Calculator for the UCC28750 only gives primary inductance and turn ratio under Transformer Design. We need to select the core size, shape, materials, and bobbin size for our current application requirements. Is there a design Calculator or document that can be used to select the transformer core for this IC? If available, kindly share the required files.

Regards,

Punith